(a) Field of the Invention
The present invention relates to a method of laser welding. More particularly, the present invention relates to a method of laser welding where two steel sheets are welded by a laser beam after the two steel sheets are overlapped with a predetermined gap.
(b) Description of the Related Art
Generally, a spot welding is mainly used as a welding method for assembling components of a vehicle body, and an arc welding or a laser welding is used at welding portions to which the spot welding cannot be applied.
Particularly, since the laser welding has merits such that a filler metal is not required, a welding speed is fast, a welding cycle is short, a heat input is low, a heat affected zone (HAZ) is small, and a deformation is small, the laser welding is increasingly used for welding the components of the vehicle body instead of the spot welding and the arc welding.
A keyhole welding using energy reflection and absorption in a focal region of a laser beam is mainly used as the laser welding. The keyhole welding is done in the focal region at which the laser beam is collected to welding materials by a lens and energy reflection and absorption occur.
That is, the laser beam collides with the welding materials in the focal region where electromagnetic wave energy of the laser beam is collected, and collision energy is converted into heat energy such that keyhole of the electromagnetic wave occurs. Accordingly, the keyhole welding is performed. The keyhole welding is a welding performed by making small hole at a molten pool due to vapor pressure when laser welding.
A laser welding system, shown in FIG. 1, welds steel sheets or plated steel sheets.
According to the laser welding system, a laser head 5 is mounted at a front end of an arm 3 of a robot 1, and the laser head 5 is connected to a laser oscillator 7 through a optical fiber.
The robot 1 is operated by a robot controller C, and the laser head 5 is moved along a welding portion of the welding material 9 by the robot 1. Therefore, the laser head 5 irradiates a laser beam LB and performs the welding process.
Recently, the components of the vehicle body are made of galvanized steel sheets as well as steel sheets. The galvanized steel sheet is a steel sheet on which zinc is plated. The galvanized steel sheet is classified into a hot dipped galvanized steel sheet and an electrolytic galvanized steel sheet according to manufacturing method. The galvanized steel sheet is increasingly used because rust that is a drawback of steel sheet is prevented. In addition, the galvanized steel sheet is economically feasible and has a high strength.
In a case that a predetermined gap is not formed between an upper and a lower galvanized steel sheets 11 and 12 when a welding is performed by using the laser beam after two galvanized steel sheets 11 and 12 are overlapped as shown in FIG. 2, explosive pores are formed at the welding portion W due to zinc gas generated by evaporation of zinc layer 13.
Accordingly, it is very important in order to weld the galvanized steel sheets 11 and 12 overlapped with each other by the laser beam that the predetermined gap between the upper and lower galvanized steel sheets 11 and 12 is maintained and the zinc vapor is discharged.
If the gap between the galvanized steel sheets 11 and 12 is small or large when the galvanized steel sheets 11 and 12 are welded by the laser beam, breakage of the welding portion W occurs and pin holes (microholes) due to sink of a welding bead is generated. Such pin holes are major causes of interfacial fractures at the welding portion W in tensile test.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.